1. Field of the Invention
The present invention relates generally to a multi-purpose wafer holder for holding a wafer (or other substrate) during processing in a plasma system. More specifically, the holder includes at least one stacked layer to provide clamping, conduction, heating and/or cooling. Examples of layers include an electrostatic chuck, a multi-zone He gas delivery system, a multi-zone resistance heater, and a multi-zone cooling system.
2. Description of the Background
It is known within the semiconductor processing field to use resistance heaters to heat a semiconductor wafer that is in the presence of a processing gas. Heating alters the characteristics of the reaction process occurring on the semiconductor wafer. For example, such resistance heaters have been used within quasi-hot wall, or warm wall, reactors where the resistance heater serves as the support for the silicon wafer and simultaneously heats the wafer to carry out a predetermined processing step. Often, a processing gas of a predetermined purity and/or pressure is circulated over the heated silicon wafer to modify the surface characteristics of the silicon wafer. Chemical vapor deposition is one environment in which such resistance heaters are used to process semiconductor wafers.
Such resistance heaters have typically employed heating elements of (1) a nickel-chromium alloy (nichrome) or (2) an aluminum-iron alloy, which are electrically resistive and which generate heat when an electrical current is applied through the elements. Examples of commercially available materials commonly used to fabricate resistive heating elements employed in ovens are Kanthal, Nikrothal and Alkrothal, which are registered trademark names for metal alloys produced by Kanthal Corporation of Bethel, Conn. The Kanthal family includes ferritic alloys (FeCrAl) and the Nikrothal family includes austenitic alloys (NiCr, NiCrFe).
However, resistance heater elements have not, in the past, been exposed to the processing gas that is circulated over the silicon wafer within the reactor. Moreover, large heater elements with large thermal masses have been required to provide a barrier of material between the wafer and the heater elements when heating in known plasma systems. Known electrically resistive materials, such as some of the Kanthal alloys, require an oxygenated environment for long element life. The presence of oxygen causes an aluminum oxide to form upon the surface of a Kanthal alloy heater element which inhibits evaporation of the heater element. An acceptable level of oxygen is 5% of 760 torr with no other gases which react to the alloy surface. Alternatively, environments with less oxygen can cause the oxide layer to become porous and allows iron oxide to migrate along grain boundaries, causing contamination of the system.
Traditionally, the heater elements of wafer processing systems also have had thermal masses that are significantly greater than the wafer or substrate that they have heated. In known systems, heater elements weighing tens of pounds have been used to heat wafers weighing only two ounces. As a result of the large thermal mass, the heater has a pronounced lateral profile which heats a wafer much more in the center region than compared to the edge region of the wafer. In order to compensate for the thermal profile, complicated devices having on the order of 30 parts were used in known systems to adjust the applied heat. A large thermal mass also leads to a high thermal inertiaxe2x80x94an effect where the heater element continues to apply heat to the wafer (or other substrate) after the wafer has already achieved its desired temperature. Also, in known systems, radial or lateral heat transfer has been higher than in the substrate being processed, making it harder to isolate changes in temperature in the wafer.
Accordingly, it is an object of the present invention to provide a wafer holder that is fabricated as a stack of at least one element, each element performing at least one wafer processing function.
It is a further object of the present invention to provide an improved wafer chuck using electrostatic clamping to provide a more uniform thermal conduction of a wafer than a non-electrostatically clamped wafer.
It is yet another object of the present invention to measure the effective clamping of a wafer to the electrostatic chuck by measuring the capacitance of (1) the wafer and (2) two adjacent electrodes housed inside the chuck that provide the clamping.
It is an additional object of the present invention to provide an He gas delivery system (known as a Helium back side) for increasing conduction to a wafer.
It is a further object of the present invention to provide a multi-zone He gas delivery system for increasing conduction in one region of a wafer more than in another region of the wafer by providing different pressures in the different zones.
It is another object of the present invention to provide a resistance heater for heating a semiconductor wafer within a wafer processing reactor wherein the processing gas circulated about the wafer is isolated from the resistance heater element.
It is another object of the present invention to provide a resistance heater which utilizes materials such as Kanthal alloys, Hastaloy, platinum, and molybdenum.
It is yet another object of the present invention to provide an oxygenated environment for those high resistance heater elements which would otherwise degrade in a low oxygen environment.
It is still another object of the present invention to provide such a resistance heater having multiple heating zones for better control of temperature uniformity.
It is a further object of the present invention to provide a resistance heater where the gas environment surrounding the heater element differs from and is completely isolated from the gas environment within the semiconductor wafer reactor.
It is another object of the present invention to measure the heat transfer characteristic of a substrate to be heated and to provide a resistance heater which is designed to apply additional heat to areas which have higher heat loss.
It is a further object of the present invention to provide a resistance heater configured to provide uniform heating across a non-circular element to be heated.
It is an additional object of the present invention to provide a resistance heater with a thermal mass closely equivalent to the thermal mass of the wafer to be heated.
It is an object of the present invention to provide a cooling system to reduce the temperature of a substrate before, during or after a plasma process.
It is an additional object of the present invention to provide a multi-zone cooling system that cools a wafer before, during or after a plasma processing step according to a heat loss pattern of the wafer.
It is another object of the present invention to provide a combined stack of more than one of the above elements (i.e., more than one of a multi-zone electrostatic chuck, a multi-zone He gas delivery system, a multi-zone resistance heater, and a multi-zone cooling system).
Briefly described, and in accordance with a first embodiment, the present invention relates to a stack of elements onto which a substrate (e.g., a wafer or an LCD panel) can be placed during a series of one or more plasma or thermal processing steps. The types of elements in the stack include, but are not limited to: an electrostatic clamp (either single- or multi-zone), a He gas delivery system (either single- or multi-zone), a resistance heater (either single- or multi-zone), and/or a cooling system (either single- or multi-zone). At least one of the elements is selected based on the processing step(s) to be performed. Each element is hermetically sealed from each other element and from the processing environment. Accordingly, one embodiment of the present invention acts as more than one of: an electrostatic chuck with electrostatic clamping, a resistance heater, and a cooling system.
According to one embodiment of the present invention, an electrostatic chuck is provided which clamps a substrate thereto and enables the clamping of the substrate to be measured. In this embodiment, first and second electrodes are housed internal to the electrostatic chuck and clamp the substrate to the chuck. The capacitance between the first and second electrodes is measured after the substrate is applied to the chuck to determine the effective clamping of the substrate.
According to another embodiment of the present invention, plural sealed plates are used together to provide pressure control for an He gas delivery system on the back side of the substrate in a plasma processing chamber. The Helium gas delivery system provides He to the back side of a substrate at a pressure significantly greater than the chamber (processing) pressure (i.e., typically 30-50 Torr) in order to improve the conduction of heat between the substrate and the chuck. Electrically clamping the substrate to the chuck enables using a back side gas pressure substantially greater then the chamber pressure. The area of the substrate and chuck are smooth enough that they provide a good gas seal. In one embodiment, the He gas delivery system is zoned to provide different amounts of He to different parts of the substrate in order to match a conduction profile of the substrate. The He gas delivery system may be used in combination with the above-described electrostatic chuck. In one embodiment of the combination, the electrostatic chuck includes gas holes and the He gas delivery system is placed below the electrostatic chuck. The He gas then passes through the holes in the electrostatic chuck to provide conduction to the wafer. The gas conduction varies directly with pressure (e.g., up to 15 torr).
According to another embodiment of the present invention, a resistance heater is provided for heating a semiconductor wafer within a wafer processing reactor. The heater includes a resistance heater element disposed in a heater channel formed in one or more quartz plates. One embodiment of the resistance heater includes heater channels with supply end head room to accommodate expansion and contraction of the heater element which occurs during heating and cooling.
The quartz plates forming the housing for the resistance heater are fused together at plural adjoining surfaces and preferably at all adjoining surfaces. The resistance heater element is secured therebetween to form a gas-tight chamber. A sufficient number of fusion points are provided to prevent the internal pressure of the resistance heater from popping the fusion points when in the reduced pressure atmosphere of the plasma processing chamber. Electrical terminals coupled to the resistance heater element are provided for conducting electrical current. In at least one embodiment, a gas duct is connected to the gas-tight chamber formed between the quartz plates for admitting gas(es) of predetermined composition(s) and pressure(s) thereto independent of the composition and pressure of the processing gas circulating about the external face of the wafer holder.
Preferably, the resistance heater element is formed of a material such as a Kanthal alloy or platinum, since those materials can be heated in air without damage. A sheet of material may be melted, drawn, chemically etched, sputtered, laser cut, cut with a water jet, or otherwise shaped to form a resistance heater element matching the heat transfer characteristics of the element to be heated. Alternatively, one or more wires of one or more of the above materials may be used as the heater element. The quartz plates are constructed to have features matching the heater elements. The features, when fused together, provide a hermetic seal to separate the processing environment from the gas over the conductor. The seals must reliably accommodate the ultra-high temperatures of the heater.
To provide a resistance heater element with a shape corresponding to the heat loss of the element to be heated, the thermal transfer characteristics are examined using at least one of the three disclosed techniques. In a first technique, changes in an LCD paper are examined when the LCD paper is applied to a previously uniformly heated substrate which is placed on an electrostatic chuck. Photographs of the changes in the paper indicate the shape of the heat loss of the heated substrate. In a second technique, an infrared scanner or detector measures the changes in heat emissions across the surface of a previously heated substrate as it cools on the chuck. One such detector that monitors spatially and temporally evolving temperature in the absence of a plasma is a commercially available wafer instrumented with thermocouples (e.g., SensArray Corporation, Model No. 1530A). In the third technique, the transfer characteristics of a substrate on a chuck are simulated by a computer based on the shapes and thermal characteristics of the substrates to be heated and the shape and heat transfer characteristics of the chuck on which the substrate is heated.
Also provided according to the present invention is a set of cooling plates for one or more of: (1) cooling a substrate before processing, (2) maintaining a cool temperature of the substrate during processing and (3) cooling the substrate after processing is complete. One embodiment of the cooling system is a multi-zone cooling system that cools a substrate according to a heat loss characteristic of the substrate. By applying coolant more rapidly in areas that would otherwise cool more slowly, the substrate is more uniformly cooled. Also, by reducing the substrate temperature quickly after processing, any temperature-based reactions are more effectively halted at a process end-point.